Method of generating force between a structure and an additional member

ABSTRACT

A method of generating force between a structure and an additional member, particularly for reinforcing an existing structure so that an additional member can share existing load. Pre-cooled or pre-heated shims of shape memory alloy (SMA) are inserted between the additional member and the existing structure, or between different sub-units of the additional member, or between the additional member and shoes fixed for this purpose to the existing structure. On subsequent warming or cooling the SMA shims expand to apply compressive forces to the portions of the structure, additional member or shoe(s) with which they are in contact on their opposite sides. Further compressive, tensile, flexural or torsional forces will be developed within other parts of the structure and additional member as a result of the shim expansion depending on the particular arrangement. Alternatively the SMA may be of a type which expands when warmed above ambient temperature and remains in the expanded state despite cooling back to ambient.

This application is the US national phase of international applicationPCT/GB01/01977 filed 8 May 2001, which designated the US.

BACKGROUND OF THE INVENTION

This invention relates to a method of reinforcing structures and hasparticular but not exclusive application to reinforcing offshorestructures such as oil and gas rigs.

It is often required to increase the load that existing structures suchas oil rigs can carry. Typically such structures are constructed fromlarge steel tubes.

One way of reinforcing the structure is by welding on new structures oradditional members. However, for a structure that is already close toits loading limit there is a fundamental difficulty that only theadditional load that is subsequently applied can be shared by the newstructure; the existing load will be carried entirely by the existingstructure. One way to avoid this difficulty is to remove all or part ofthe load when carrying out the retrofit. Upon re-loading, the newstructure will take a greater share of the overall load than it would ifthe load could not be removed. Normally, for large structures, there islimited scope for removing load, so that optimum load sharing is notachievable. Consequently, the scope for carrying additional load may bevery limited.

In addition, in the case of oil and gas rigs that are required to remainoperational throughout any retrofits, welding or other operationsinvolving the use of naked flames, are not allowable.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of reinforcingstructures without the need to remove the load and that obviates thenecessity of welding and which furthermore allows for the existing loadto be shared by new additional structures or members.

The invention comprises a method of reinforcing a structure with anadditional member, said additional member comprising one or moresub-units comprising the steps of: a) inserting pre-cooled or pre-heatedshims of shape memory alloy between said member and said structure orbetween at least two of said sub-units; b) allowing said shims to expandon subsequent cooling or warming so as to force said member intocompression or tension against said structure, also driving an adjacentportion of said structure into tension or compression. Shape memoryalloys are also possible in which the transition temperature changesafter activation. A shim made from such an alloy could be inserted atambient temperature, caused to expand by heating, and remain in theexpanded state on cooling to ambient temperature and below. Such analloy is Ti—Ni—Nb.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described with reference to the following figuresof which:

FIGS. 1 a to e show how the intersection between hollow tubes can bereinforced according to one embodiment of the invention.

FIGS. 2 a to c show how an additional reinforcing member can be attachedto a pre-existing structure to share loading.

FIGS. 3 a to f show alternative embodiments of how an existing membercan be reinforced by external clamping.

FIGS. 4 a and b show how an entirely new member can be introduced insuch a way as to share some existing load within the structure or tocarry load that was previously carried by a bent or damaged member.

FIG. 5 shows how load can be transferred into a thick member in order toallow reinforcement elsewhere in the structure.

FIGS. 6 a to l show embodiments of load carrying shoes for use at andaround tube intersections according to the invention.

FIG. 7 shows how a complete compressive tubular structure might bereinforced using the techniques described.

FIGS. 8 a and b show how a sheet of shape memory alloy (SMA) can changeshape on passing through its transition.

FIG. 9 shows how a wall or building might be underpinned using thetechniques described.

FIGS. 10 a to c show how a tensile member can be reinforced using thetechniques described.

FIGS. 11 a to c show how a member subject to torsional loading can bereinforced using the techniques described.

FIGS. 12 a to 13 b show how a member subjected to bending can bereinforced using the techniques described.

FIGS. 14 a to d show alternative clamping arrangements for thereinforcement of an existing member.

FIG. 15 shows an embodiment of the invention reinforcing pre-stressedconcrete.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 a shows schematically one of the most common failure modes (shearfailure) in a structure 1. This is caused by punching through of adiagonal strut 2 to a vertical hollow member 3. One solution would be tofill the large tube with concrete or other appropriate material. Howeverthis would render the whole structure uninspectable from the inside. Inaddition as the enhanced load is transferred to the vertical tube afterinstallation of the retrofit, the diameter thereof would increase due toPoissons ratio. Thus the inside wall would lose contact with theconcrete, so much of the support would be lost.

In the FIG. 1 b embodiment of the invention, the problem is solved byinsertion of a supporting ring 4 which is installed inside the verticaltube as seen in FIG. 1 b. FIG. 1 c shows a cross-section through A—A ofFIG. 1 b. The support ring comprises a number of steel segments 5,between each segment is inserted a pre-cooled shape memory alloy shim 6,as seen in FIG. 1 d. For example, these shims might be formed and storedin liquid nitrogen. After warming up the shims expand (shown in FIG. 1e) and the diameter of the supporting ring therefore increases, formingan interference fit within the vertical member 3. As the supporting ring(comprising a number of items 5 and 6) is compressively loaded aroundthe circumference, a ring of discrete segments can suffice. In effect,the supporting ring acts as an internal clamp. Normal thermal expansionof materials is insufficient to provide the necessary displacements forthis mechanism to operate effectively.

In FIGS. 1 c to e the shims are shown as having constant thickness, butwedge shaped, gently curved or other shaped shims may also be usedaccording to the situation.

It would be clear to the skilled person that the number, size andthickness and choice of materials can be made according to the exactrequirement.

Shape memory alloys are a class of metallic alloys that have the abilityto undergo an apparent plastic transformation at lower temperature whichcan be recovered on heating above a certain temperature. These includealloys based on the systems Ti—Ni, Cu—Zn—Al, Cu—Al—Ni, and Ti—Ni—Nb butare not limited to these systems. One such alloy comprises 44 atomic %Ti, 47 atomic % Ni and 9 atomic % Nb.

In the above descriptions the term “shim” has been referred to. In theremainder of this specification, this term should be construed asincluding the term “spacer”. Such a shim may also comprise a stack ofsheets.

FIG. 2 shows an alternative example of how an existing structure can bereinforced by additional members to take a share of the existing as wellas an extra load. The diagram shows a portion of a structure includingtwo existing large vertical members 3 and an existing diagonal member 2.It is assumed in this example that member 2 is the weakest part of thestructure and requires reinforcement. In order to reinforce thestructure shoes 7 are fixed, by appropriate means such as bolting on,onto the main tubes at each end in the region of the diagonal member. Anauxilliary member 8 comprising two half tubes is bolted together aroundthe diagonal member. A cooled shape memory alloy shim 6 is then insertedbetween the auxilliary member and one of the shoes. On expansion whenbecoming warm, the additional auxilliary member (comprising the halftubes) are then driven into axial compression, thereby relieving some ofthe compressive load in the existing member. It is often only necessaryto apply the SMA shims at one end or other of the member. For largestructures it will normally be most convenient to insert these at theupper end of the member where the gap will naturally occur with theauxiliary member resting on its lower end. However, there may becircumstances in which it is more convenient to use a shim at each endof the auxiliary member. An example of this is where by using a thinshim at each end the heat transfer from the structure, and hence theactivation of the SMAs, is speeded up.

The auxiliary member 8 can consist of more than two pieces if this ismore convenient for lifting into place and assembly. For example itmight consist of three pieces. FIGS. 2 b and 2 c are sections throughB—B showing two and three piece versions. The assembly of old membersurrounded by the new members may also be constrained radially by anoverwind, bandage or series of circumferential clips in order tostabilise it against buckling.

FIG. 3 shows alternative examples of how a tubular member 9 can bereinforced. In the FIG. 3 a embodiment an additional sleeve 10 isinserted around the member and cooled SMA shims (spacers) 6 are insertedbetween the existing tube and additional sleeve. On warming the shimswill force the existing tube member into compression. This is a methodof either providing additional reinforcement against internal pressureloading or as a means of tightly clamping on a collar capable ofwithstanding a high load applied along the axis of the existing tubemember.

FIG. 3 b shows a similar embodiment wherein instead of a sleeve, abracelet/clamp arrangement 11 is fixed around existing tube 9. Onwarming the SMA shims expand so as to force the existing tube of thestructure into hoop compression and the bracelet into tension. On alarge structure, the clamping forces developed can be larger than wouldbe achievable by simply tightening the screws.

FIG. 3 c shows a similar embodiment wherein the clamp comprises twooverlapping portions 12. This has some advantage over the FIG. 3 bembodiment as it can take more tension than that sustainable by thebolts of the FIG. 3 b bracelet.

FIG. 3 d shows an alternative embodiment in which the parts of the clampare held together by pins. The two parts of the clamp are interleavedwhere they overlap 13 and a pin 14 is inserted through the holes. Thisarrangement is capable of achieving a very strong clamp. If a longerclamp is needed, that is one extending further along the length of themember 9, further interleaving is clearly possible. The idea of frequentinterleaving is to ensure that the pins are loaded in shear rather thanin bending.

FIGS. 3 e and f show an arrangement with three clamp pieces and threepins 14. This may be more convenient for lifting into position and foraccommodating non-circularity in the tube to be clamped. Clearly clamparrangements consisting of even more components are possible.

It should be noted that additional members 15 (see FIG. 4 a) can beadded to reinforce a structure at any suitable location even where amember does not already pre-exist, and the expanding shim principle usedto transfer load into such additional members. In the above example ofFIG. 2, if there was not already a pre-existing diagonal member, onecould be added by having shoes fitted on to the two vertical tubes andthen inserting the additional diagonal member between the shoes. Cooledshape memory alloy shims would then be inserted between the shoes andthe ends of the additional member. On warming the member will be driveninto a state of axial compression allowing it to take the desiredportion of the pre-existing load. This is shown in FIG. 4 a.

In another simple embodiment the additional member can be insertedbetween portions of the existing structure to relieve the load in a bentor otherwise damaged member. On warming up the shim will expand to forcethe additional member into compression against the structure. Againappropriately shaped shoes which snugly fit the portion of structure andend of the additional member may be advantageously required. This isshown in FIG. 4 b.

The external clamps of the type shown in FIGS. 3 b to f may be used forload transfer into a main member. FIG. 5 shows the method by whichreinforcement can be given to a relatively thin member 3 mounted on athick member 17. A ring of SMA shims 6 is clamped 16 to the thick member17. In this example, compressive load might be transferred into theauxiliary member 18 by means of SMA shims (not shown) at its upper end,in order to share load with the vertical member 3.

The design of the shoes 7 in FIG. 2 will normally be multi-part becauseof the need to assemble them around the intersection points. FIGS. 6 ato l show some examples of such shoes.

FIGS. 6 a and b show front and corresponding side views of a shoe 19split vertically and held together by horizontal pins or screws 20. Thisarrangement presents a surface 21 normal to the axis of the diagonalmember against which the SMA shim (not shown) can bear.

FIGS. 6 c and d show front and corresponding side views of a similarshoe split into upper and lower parts which are pinned or screwedtogether.

FIGS. 6 e and f show front and corresponding side views of a shoearrangement suitable for reinforcing the vertical member as well as thediagonal member. Thus auxiliary members (not shown) can bear againstsurfaces 21,22 and 23.

FIGS. 6 g and h show front and corresponding side views of a similararrangement in which the two parts of the shoe are held together byexternal clamps 16, including any of types shown in FIGS. 3 b to f,instead of or as well as pins and screws.

FIGS. 6 i and j show front and corresponding side views of anarrangement in which the shoe 19 bears against a vertical clamp 16. Theload is transferred to the vertical member 3 through the clamp andthrough the “elbow” of the intersection 24. This arrangement has theadvantage that the shoe parts do not necessarily have to be a good fitaround the intersection.

FIGS. 6 k and l show front and corresponding side views of anarrangement suitable for an intersection between a vertical member 3 andtwo diagonal members that allow all three to be reinforced throughbearing on surfaces 21, 22 and 23.

In principle, a whole tubular structure can be reinforced using thepreviously stated principles. FIG. 7 shows an example of this. It willbe apparent that approximately balanced loading from the variousdirections, as occurs on items 25 and 26, allows a lighter design ofshoe than if the loads are unbalanced.

When a shape memory alloy is activated, its volume remains approximatelyconstant. It follows that a large expansion in one direction t must beaccompanied by a contraction in one or more of the other directions x,y. FIG. 8 a shows an SMA sheet which is below its transitiontemperature. FIG. 8 b shows the same SMA sheet above its transitiontemperature. It follows that it is possible to select a direction inwhich the shim would expand on cooling, and this may be more convenientfor some purposes. Shape memory alloys with higher transitiontemperatures are also feasible, so that compositions that can be formedand stored at temperatures higher than liquid nitrogen temperatures arepossible.

All the examples described above refer to structures consisting largelyof circular tubes. It will be apparent to a skilled person that many ofthe principles are applicable to non-circular tubes or solid sections;particularly the type of reinforcement shown in FIG. 2.

Another example is the use of SMA expanding shims for underpinning awall 27 or part of a building, as shown in FIG. 9. Supporting beams 28and 29 can be jacked apart using SMA shims 6 to achieve a prescribedvertical displacement. Such shims, when activated, can remain in placepermanently or be removed once a suitably strong filler has beeninserted in the gap 30.

Another example is the use of SMA expanding shims to reinforce a tensilemember within a structure. FIGS. 10 a and 10 b show a lateral frontelevation and a cross-section through C—C respectively of a member 31under tensile load imparted to it by two other members 32. Here themethod of reinforcement is to introduce a tensile reinforcing member 33,which for the purposes of assembly is split, as in previous examples,and held together at 34 with pins or bolts. The new member embraces themembers 31 and 32 in such a way that SMA shims 6 can be inserted in anappropriate gap at one or both ends. When activated, the SMA shimtransfers compressive load through an adapter shoe 35 between member 32and the new member 33. In this way member 33 is driven into a state oftension, thereby relieving the tension in member 31.

The interleaved, pinned joint shown in FIGS. 10 a to c is only one ofmany methods by which the parts of member 33 are held together. Otheralternatives are a strap or overwind over the cylindrical section of 33or simple bolted joints.

The purpose of the adaptor shoe 35 is to provide a second flat surfacefor the shim to bear against. Its other surface conforms to that ofmember 32. If member 32 were of rectangular section then item 35 mightbe dispensed with.

The same reinforcement principle is applicable to an oblique tensilemember. FIG. 10 c shows a method of achieving this. The adapter shoes 35are modified to provide a surface for the SMA shims to bear against,which is perpendicular to the axis of member 31. Member 35 in thissituation will be subjected to a component of load along the directionof member 32 (downwards in the figure). If member 31 is almostperpendicular to member 32 (that is the degree of obliquity is small)then friction between members 35 and 32 may be sufficient to transferthis load. If however, the obliquity angle is larger then member 35 willrequire a feature to bear against. This might be accomplished by fixingitem 35 securely to item 33. The axial load would then be transferredthrough the junction between members 32 and 33. This might, however,introduce an unacceptably large shear load at that junction. Analternative shown in FIG. 10 c is to extend item 35 so that it bears ona collar 36. This collar may be of conventional design or any of theclamps shown in FIG. 3.

Another example is the use of SMA expanding shims to reinforce a membersubject to torsional loading within a structure. FIGS. 11 a and 11 bshow lateral front and side elevations of a member 37 under torsionalload imparted to it by two other members 38. FIG. 11 c shows across-section through D—D (FIG. 11 b). Here the method of reinforcementis to introduce a torsional reinforcing member 39, which for thepurposes of assembly is split, as in previous examples. The new memberhas extended lobes 40 such that SMA shims 6 can be inserted inappropriate gaps at some distance off the axis of member 37. Adaptershoes 35 would be used as in the previous example. When activated, theSMA shims apply a torsional load to member 39 through lobes 40, therebyrelieving some of the torsional load in member 37.

In order to withstand torsional loading the two parts of member 39 haveto be secured to avoid shearing at the interface between them. In FIG.11 a, this is accomplished by inserting dowel pins 41, which would berequired at various stations along member 39. It will be evident to aperson skilled in the art that one of a number of methods might be usedto key the two parts of member 39 together.

Another example is the use of SMA expanding shims to reinforce a membersubject to bending loads within a structure. FIGS. 12 a and 12 b show alateral front elevation and a cross-section through E—E respectively ofa member 42 under bending load imparted to it by two other members 43.Here the method of reinforcement is to introduce an additionalreinforcing member 44, which for the purposes of assembly is split, asin previous examples. The new member has extended lobes such that SMAshims 6 can be inserted into appropriate gaps at some distance off theaxis of member 42. Adapter shoes 35 would be used as in the previousexample. When activated, the SMA shims apply a bending load to member 44thereby relieving some of the bending load in member 42.

Member 44 does not necessarily have to extend the full length of member42. FIGS. 13 a and 13 b show a lateral front elevation and a plan viewrespectively of member 44 split into two separate islands, each islandthen being split as in previous examples. The corrective bending loadscan then be applied to member 42 by virtue of members 44 fitting closelyaround it in the regions where the load is applied. The arrows show theset of corrective forces exerted on the existing structure.

FIGS. 14 a to d show further embodiments utilising the clamping concept.FIG. 14 a shows a clamp 45 around a pipe 46, without the use of SMAshims. In general, SMAs are capable of exerting considerably morecompressive stress than is needed to activate the clamp (for thin walledtubes, typically by a factor of 20). Thus if used to apply uniformpressure around the circumference of a pipe 46, as shown in FIG. 14 b,the SMA 6 is capable of collapsing the pipe in compression or breakingthe clamp 45 in tension. To avoid these eventualities, the design wouldhave to ensure that the SMA was unable to develop sufficientdisplacement to do this (i.e. it would run out of stroke). Using SMA allround the pipe, however, is somewhat wasteful of the relativelyexpensive SMA material.

A more economical use of the SMA is as discrete “islands” of shim 6, asin FIG. 14 c. Here the average pressure (averaged around thecircumference of the pipe) is sufficient for effective clamping, butboth the clamp 45 and the pipe 46 are subject to bending. To compensatefor this, the clamp ring would have to be thicker than would otherwisebe the case. In addition the pipe is more difficult to grip.

The use of inert packing pieces 46, as well as the active SMA shims 6,as shown in FIG. 14 d, provides a partial solution to this. The bendingloads in the clamp are partially relieved, and the pressure applied tothe pipe is more uniform.

FIG. 15 shows how the principle of reinforcing a tensile member can beused to construct components in materials such as pre-stressed concretein the field, without the need for jacking devices.

The reinforcing rods 47 are set in a strong frame 48, through whose endsthey pass. The ends of the rods are secured, e.g. by nuts and washers orwelded on lugs 49. SMA shims 6 are inserted between items 49 and one orboth ends of the frame. When the SMA is activated, the rods are drawninto tension. The concrete is then cast into a box or shuttering, item50.

After the concrete has set and the box and frame have been dismantled, apre-stressed reinforced concrete beam results.

1. A method of generating force between a structure and an additionalmember, said additional member comprising one or more units, said methodcomprising: a) inserting one or more pre-cooled or pre-heated shim(s) ofshape memory alloy between said additional member and said structure or,inserting said shim(s) between at least two of said units, or,additionally fixing one or more shoes to said structure and insertingsaid shim(s) between said shoe(s) and said additional member; each saidshim being a generally laminar element adapted to expand in thickness onwarming or cooling through a transition temperature; and b) allowingsaid shim(s) to expand in thickness on subsequent warming or cooling soas to apply a compressive force to respective portions of saidstructure, additional member or shoe(s) in contact with the oppositesides of respective said shim(s).
 2. A method as claimed in claim 1wherein said additional member has an outer surface that generallyconforms to the shape of the inner surface of an existing structuralmember of said structure.
 3. A method as claimed in claim 2 wherein saidexisting structural member is tubular.
 4. A method as claimed in claim 2wherein in step a) the shim(s) are inserted between the inner surface ofsaid existing structural member and the outer surface of the additionalmember.
 5. A method as claimed in claim 2 wherein said additional membercomprises at least two sub-units, said shim(s) being inserted betweensaid sub-units.
 6. A method as claimed in claim 1 wherein saidadditional member has an inner surface that generally conforms to theshape of the outer surface of an existing structural member of saidstructure.
 7. A method as claimed in claim 6 wherein said additionalmember is a hoop.
 8. A method as claimed in claim 7 wherein theadditional member comprises an external bracket or clamp.
 9. A method asclaimed in claim 1, wherein said structure is a tubular, offshorestructure.
 10. A method as claimed in claim 1 wherein said additionalmember is attached to said structure by virtue of the forces generatedby expansion of said shim(s).
 11. A method as claimed in claim 10wherein said additional member reinforces said structure such as toshare existing load in said structure.
 12. A method as claimed in claim11 wherein said structure is a tubular offshore structure.
 13. A methodas claimed in claim 1 wherein said structure is part of a building andsaid shim(s) are inserted between a pair of beams located beneath saidstructure to underpin the structure.
 14. A method as claimed in claim 1,wherein said structure is a frame surrounding a space for concrete to becast and a plurality of said additional members comprise rods extendingthrough said space and frame which are placed into tension by theexpansion of shims inserted between said frame and abutments on saidrods, prior to the setting of concrete within said space.
 15. A methodof generating force between a structure and an additional member, saidadditional member comprising one or more units, said method comprising:a) inserting one or more shim(s), at ambient temperature, of a shapememory alloy between said additional member and said structure, or,inserting said shim(s) between at least two of said units, or,additionally fixing one or more shoes to said structure and insertingsaid shim(s) between said shoe(s) and said additional member; each saidshim being a generally laminar element adapted to expand in thickness onwarming through a transition temperature and to remain in the expandedstate on cooling to ambient temperature; and b) causing said shim(s) toexpand in thickness on subsequent warming so as to apply a compressiveforce to respective portions of said structure, additional member orshoe(s) in contact with the opposite sides of respective said shim(s),wherein said shim(s) remains in the expanded state on cooling to ambienttemperature.
 16. A method as claimed in claim 15 wherein said additionalmember has an outer surface that generally conforms to the shape of theinner surface of an existing structural member of said structure.
 17. Amethod as claimed in claim 16 wherein said existing structural member istubular.
 18. A method according to claim 15 wherein said additionalmember has an inner surface that generally conforms to the shape of theouter surface of an existing structural member of said structure.
 19. Amethod as claimed in claim 18 wherein said additional member is a hoop.20. A method as claimed in claim 19 wherein the additional membercomprises an external bracket or clamp.
 21. A method as claimed in claim15, wherein said structure is a tubular, offshore structure.
 22. Amethod as claimed in claim 15 wherein said additional member is attachedto said structure by virtue of the forces generated by expansion of saidshim(s).
 23. A method as claimed in claim 22 wherein said additionalmember reinforces said structure to share existing load in saidstructure.
 24. A method as claimed in claim 23 wherein said structure isa tubular offshore structure.
 25. A method as claimed in claim 15wherein said structure is part of a building and said shim(s) areinserted between a pair of beams located beneath said structure tounderpin the structure.
 26. A method as claimed in claim 15, whereinsaid structure is a frame surrounding a space for concrete to be castand a plurality of said additional members comprise rods extendingthrough said space and frame which are placed into tension by theexpansion of shims inserted between said structure and abutments on therods, prior to the setting of concrete within said space.